This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The project we propose involves the characterization of the response of fast photodetectors for the development of Time-Of-Flight Positron Emission Tomography imagers. A good deal of effort has been placed in recent years in understanding the effect of the rise time and quantum efficiency of fast PMTs on the timing resolution they afford when coupled to a fast scintillating crystal. Most of the work done has foccused on the characterization and optimization of the timing resolution afforded by the most basic detector - a single small crystal coupled to a fast PMT. When such detectors are tiled to achieve large area coverage, as is required in a PET scanner or particle detector, the measured timing resolution degrades considerably as compared to the stand alone detector. Our investigation will attempt to understand the source of this degradation in timing resolution, and devise a data acquisition and calibration scheme to minimize it. The successful investigation will establish the correlation between photodetector gain, rise time, quantum efficiency and signal shape and the large area detector timing resolution. A major part of our effort will involve the characterization of the single-photon response of different photodetectors and the variation in that response across the face of the photodetector. This variability is suspected of being greatly responsible for the degradation in timing resolution in a large-area detector. This work is supported by NIH grants 5R01CA113941-02 and 1R21EB008142-01